OCT scan: what it is and what to expect

OCT stands for optical coherence tomography. It is a scan that uses light to produce a cross-section picture of the retina and the optic nerve, showing their layers and measuring their thickness.

In brief

• What it is: a light-based scan of the back of the eye. No X-rays, no injection, no contact with the eye.
• What it feels like: painless. You look at a target light for a few seconds.
• How long: about ten minutes for both eyes.
• Preparation: none. Dilating drops are sometimes used.
• Who reads it: the doctor, together with your examination.

What the scan shows

The retina is the light-sensitive layer lining the back of the eye. It is thinner than a sheet of paper and made of several layers. An ordinary examination, or a photograph, shows its surface. OCT shows it in cross-section, in the way a slice through a layered cake shows each layer.

From the scan the doctor can see:

• whether the centre of the retina (the macula) is swollen with fluid, thinned or distorted
• whether there is fluid under or within the retina
• the thickness of the nerve fibre layer around the optic nerve, which is the layer that thins in glaucoma
• how these measurements compare with earlier scans of the same eye

OCT angiography

OCT angiography is a further kind of OCT scan available at the hospital. It detects movement of blood within the small vessels of the retina and builds a map of them, without a dye injection. It can show areas where small vessels have closed, or where abnormal new vessels have formed. It does not show leakage from vessels, which is one reason the doctor may sometimes advise other tests.

Why it is done

OCT is not a routine test for every visit. It is advised when the examination raises a question that the scan can answer, and the reason is explained before it is done.

What you will experience

1. You sit at the machine and rest your chin and forehead on a support.
2. You look at a small target light inside the machine. You may see moving lines of light as it scans.
3. You keep still and hold your gaze for a few seconds. You can blink normally between scans.
4. The other eye is scanned in the same way.

The scan is painless. Nothing touches the eye, and there is no puff of air. If dilating drops are used, those may sting for a few seconds and will blur near vision for a few hours. See before your visit for what dilation means for driving and reading.

How long it takes

Each scan takes a few seconds. Allow about ten minutes for both eyes, including positioning. A scan is sometimes repeated if you blinked or moved, which is common and not a concern.

How to prepare

• No fasting and no change to your medicines or eye drops.
• Bring any earlier scan reports, even from another hospital. Comparison over time is where much of the value lies.
• If your eyes tend to be very dry, tell the technician. Blinking a few times before each scan improves the image.
• If dilation is likely, arrange not to drive yourself home.

What the result can tell

• Whether the macula and the nerve fibre layer look normal in structure, at the level of fine detail.
• Whether fluid or swelling is present, and how much.
• Whether measurements have changed since an earlier scan.

What the result cannot tell

• It is not a diagnosis by itself. A scan is read together with your symptoms, vision, eye pressure and the examination.
• It covers a limited area. OCT images the central retina and the optic nerve, not the outer retina. A tear at the edge of the retina, for example, would not appear on it.
• It shows structure, not function. How well you see, and how wide your field of vision is, are measured by other tests such as a visual field test.
• Image quality varies. A dense cataract, a very dry eye surface, a small pupil or movement can reduce the quality of a scan, and a poor-quality scan can mislead.
• The built-in comparison has limits. The coloured "normal range" on a printout compares your eye with a reference group. Eyes that are very short-sighted or unusually shaped can fall outside that range without disease, and the reverse can also happen.
• Scans from different machines are not directly comparable. Follow-up is most reliable on the same machine.

OCT at the hospital

Dr. P. Siva Reddy Eye Hospital has OCT and OCT angiography on site, so a scan advised during a consultation is done in the same building. The doctor reads the scan together with your examination and explains it. The full list of diagnostic equipment is on the facilities and equipment page.

Frequently asked questions

Is an OCT scan painful?

No. The scan is painless. Nothing touches the eye. You rest your chin on a support and look at a small target light for a few seconds while the machine scans.

Does an OCT scan use X-rays or radiation?

No. OCT uses a beam of light. There are no X-rays and no injection.

How long does an OCT scan take?

The scan itself takes a few seconds per eye. Allow about ten minutes for both eyes, including positioning and any repeat scans.

Do my eyes need to be dilated for an OCT scan?

Not always. Many scans can be taken through an undilated pupil. If the pupil is small or the view is hazy, dilating drops may be used. If your visit includes a dilated retina examination, the scan is often done while the pupil is wide.

What is the difference between OCT and OCT angiography?

A standard OCT scan shows the structure of the retina in layers. OCT angiography uses the same kind of scan to map blood flow in the small retinal vessels, without any dye injection.

Why does my OCT scan need to be repeated?

Many eye conditions are followed by comparing scans over time. In glaucoma, change between scans matters more than a single reading. After an injection for swelling at the macula, a repeat scan shows how the eye has responded.

My OCT scan is normal. Does that mean my eyes are healthy?

A normal scan is reassuring about the area that was scanned, but it is one part of the picture. OCT images the central retina and the optic nerve. It does not examine the outer retina, measure eye pressure or test how well you see. The doctor reads it together with the examination.

Can I get an OCT scan without seeing the doctor?

The scan is a tool for the examining doctor. It is advised when the examination raises a question it can answer, and the doctor reads the result alongside the examination findings.

Related pages

• Visual field test explained
• Widefield retinal imaging explained
• Retina diagnostics
• Glaucoma
• Diabetic eye screening
• Intravitreal injection
• Before your visit
• Facilities and equipment
• Patient guide: all pages